chore: cherry-pick aeb6bc551b60 from chromium

[ppontes]

Prevent accessing shared buffers from audio rendering thread

The shared buffer in ScriptProcessorNode can be accessed by the
audio rendering thread when it is held by the main thread.

The solution suggested here is simply to expand the scope of
the mutex to minimize the code change. This is a deprecated
feature in Web Audio, so making significant changes is not
sensible. By locking the entire scope of Process() call, this
area would be immune to the similar problems in the future.

(cherry picked from commit 60987aa224f369fc0ea38c56e498389440921356)

Bug: 1174582
Test: The repro case doesn't crash on ASAN.
Change-Id: I2b292f94be65e6ec26c6eb0e0ed32b3fb2d88466
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2681193
Commit-Queue: Hongchan Choi hongchan@chromium.org
Reviewed-by: Raymond Toy rtoy@chromium.org
Cr-Original-Commit-Position: refs/heads/master@{#852240}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2715585
Commit-Queue: Krishna Govind govind@chromium.org
Reviewed-by: Srinivas Sista srinivassista@chromium.org
Cr-Commit-Position: refs/branch-heads/4324@{#2238}
Cr-Branched-From: c73b5a651d37a6c4d0b8e3262cc4015a5579c6c8-refs/heads/master@{#827102}

Notes: Security: backported fix for CVE-2021-21165.

[jkleinsc]

Merging as CI failure is flake unrelated to this PR.

[release-clerk]

Release Notes Persisted

Security: backported fix for CVE-2021-21165.